1. Technical Field
The present invention relates in general to analog signal filters and, in particular, to continuous-time filter circuits utilizing operational transconductance amplifiers. More particularly, the present invention relates to a common mode transient reduction circuit for improving the transient response of an operational transconductance amplifier and a method of operation thereof.
2. Description of the Related Art
A conventional disk drive typically includes one or more magnetic disk platters mounted for rotation on a hub or spindle. A typical disk drive also includes a transducer head supported by a hydrodynamic air bearing that hovers above each magnetic disc. A drive controller is conventionally utilized for controlling the disk drive based on commands received from a host system. The drive controller controls the disk drive to retrieve information from the magnetic disks and to store information on the magnetic disks.
An electromechanical actuator operates within a negative feedback, closed-loop servo system to position the transducer head. The actuator moves the transducer head radially over the disk surface for track seek operations and holds the transducer head directly over a track on the disk surface for track following operations. Information is typically stored in concentric tracks on the surface of the magnetic disks. A number of axially aligned tracks on the disk surfaces are collectively referred to as a cylinder. Data is written to the disk by providing a write signal to one of the transducer heads to encode flux reversals on the surface of the magnetic disk representing the data to be stored. In retrieving data from the disk, the drive controller controls the electromechanical actuator so that the transducer head flies above the magnetic disk, sensing the flux reversals on the magnetic disk and generating a read signal based on those flux reversals. The read signal is typically conditioned and then decoded by the drive controller to recover data represented by flux reversals stored on the magnetic disc and consequently represented in the read signal provided by the transducer head.
A continuous-time filter is generally utilized to reduce wideband noise and for shaping the readback data and is commonly constructed utilizing a number of tunable transconductance stages. Typically, each stage includes an operational transconductance amplifier (OTA). The cutoff frequency of a transconductance (gmC) continuous-time filter is proportional to the product of a transconductance (gm) and the inverse of a load capacitance. For read channel applications, the cutoff frequency of transconductance continuous-time filter must be programmable over at least a 3-to-1 range in two separate modes of operation; a servo mode and a read mode. The two modes of operation require that the cutoff frequency range of the filter to be at least a 5-to-1 ratio. A conventional read channel, for example, may have an overall cutoff frequency range of 20 MHz to 200 MHz. In actual operation, a fast transition in a read channel""s transconductance continuous-time filter bandwidth between a low frequency, i.e., servo, operation and a high frequency, i.e., read, operation is desirable. However, when the operational transconductance amplifiers biasing voltage is changed to increase the closed loop bandwidth of the transconductance continuous-time filter, the common mode voltage of the operational transconductance amplifiers is also perturbed. Thus, an unnecessary delay is introduced to allow the common mode level to recover from the bias shift limiting a fast transition from the servo operation to the read operation.
Accordingly, what is needed in the art is an improved transconductance continuous-time filter circuit that mitigates the limitations discussed above. More particularly, what is needed in the art is an operational transconductance amplifier with improved transient response.
To address the above described deficiencies of the prior art, and in accordance with the invention as embodied and broadly described herein, a common mode transient reduction circuit for use with an operational transconductance amplifier having a main amplifier and a common mode feedback amplifier coupled to a common bias voltage is disclosed. The common mode transient reduction circuit includes a delay circuit, coupled between the common bias voltage and the main amplifier, that reduces a magnitude of the main amplifier""s common mode voltage output transient when the common bias voltage is changed. In an advantageous embodiment, the delay circuit includes a resistance and a capacitance. In a related embodiment, the delay circuit also decreases the bias time response of the main amplifier to substantially equalize both of the bias time responses of the main and common mode feedback amplifiers when the common bias voltage is changed.
The present invention discloses a novel transient reduction circuit and method for improving the transition response period of a transconductance continuous-time filter utilizing operational transconductance amplifiers when the transconductance continuous-time filter is switched, for example, from a low frequency, i.e., servo, operation to a high frequency, i.e., read, operation. The present invention recognizes that the bias of a common mode feedback amplifier employed in a conventional operational transconductance amplifier changes before that of the operational transconductance amplifier""s main amplifier when a common bias voltage is changed, for example, when increasing the closed loop bandwidth of the continuous-time filter. With conventional filters, the transition response period for the operational transconductance amplifier""s common mode level to recover from the bias shift limits a fast transition from a servo to a read operation. The present invention utilizes the delay circuit to equalize the response of the operational transconductance amplifier""s main and the common mode feedback amplifiers when the common bias voltage is changed. Furthermore, the delay circuit of the present invention also reduces the magnitude of the operational transconductance amplifier""s common mode voltage output transient and, consequently, the transition response period required before the signal output from the transconductance continuous-time filter can be effectively utilized is reduced.
In another aspect of the present invention, a method for decreasing a transition period of a continuous-time filter from a low frequency operation to a high frequency operation, where the transconductance continuous-time filter includes at least one operational transconductance amplifier having a main amplifier and a common mode feedback amplifier is disclosed. The method includes utilizing a common bias voltage to bias the main and common mode feedback amplifiers. The method also includes introducing a delay between the common bias voltage and the main amplifier such that the magnitude of the main amplifier""s common mode voltage output transient is reduced when the common bias voltage is changed thereby decreasing the transition, or settling, period of the common mode output voltage signal.
The foregoing description has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject matter of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.